The present invention relates to filament switching devices and in particular to a switching device which is designed to be compactly mounted within the screw base of a conventional electric lamp.
It is known to operate a pair of incandescent filaments so that upon failure of a primary filament an automatic switching device substitutes a secondary filament for the failed primary. A known multi-filament lamp employs a fusible conductor which restrains a spring-leaf contact. Upon failure of a primary filament the surge of current associated with failure ruptures that fusible conductor, releasing the spring-leaf contact and causing it to substitute the secondary filament for the failed primary filament. In another known example of such devices, an insulator which separates a pair of contacts breaks down upon failure of a primary filament and allows these contacts to close and substitute a secondary filament for the failed primary filament.
These known filament switching devices have generally required significant amounts of space within the screw base of a light bulb. Accordingly, manufacturers of light bulbs would require specialized machinery to handle an enlarged screw base. Moreover, the enlargement of the screw base would result in a longer lamp or in a bulb shortened to account for the enlarged screw base. Such a change in dimension or shape can be unacceptable in certain applications. Another practical impediment to successful implementation of a filament switching device has been containment of the fragments resulting from the fusing of the fusible conductor. Known devices have not had provisions for preventing such hot fused fragments from striking the fragile glass bulb. Such inadvertent contact on the glass bulb can cause its breakage which results in air leakage. Therefore, any attempt to commercialize a light bulb having an automatically switched pair of filaments would be unsuccessful.
The present invention avoids such problems and disadvantages by mounting a cantilevered and a fixed contact within a frame having a floor which branches in two directions. Since the frame branches in two directions it can fit around the evacuation tube of a conventional light bulb and thus compactly fit within the conventional screw base of such a bulb. These contacts are employed to automatically substitute another filament for a failed filament. Accordingly, a reliable light bulb can be efficiently and inexpensively manufactured using the conventional size screw base, exhaust tubing, and dual filament mount.
Moreover, since fabrication of a dual filament light bulb is commonplace, the present switching device can be reaily incorporated into existing manufacturing lines without redesign or modification of the dual filament bulbs. Also, since its screw base need not be elongated or enlarged the lamp can maintain its standard length and shape.
Since any incandescent filament has an inherently limited life, the effective life of an incandescent lamp according to the present invention can be doubled without sacrifice in luminous efficacy. This is a significant achievement, since any appreciable extension in filament life of commercial lamps necessitates lowering of the operating temperature of the filament which inevitably decreases luminous efficacy. Therefore, the so-called long life bulbs available commercially are undesirable for energy conservation. But redundancy provides reliability, which is extremely important for applications where it is prohibitively expensive or impractical to routinely rep;ace the light bulbs. Exemplary prior art are U.S. Pat. Nos. 2,049,338 and 2,217,794.
In one embodiment, the frame of the switching device branches in two directions but is bridged by a cross-piece which thereby forms a central passageway. This passageway is sized to receive the evacuation tube so that the switching device is mechanically self-aligning and compactly mounted within the associated screw base. It is also preferable that the frame of the switching device be a pair of complimentary shells shaped to fit together and encompass the fusible conductor, thereby entrapping fused fragments which may issue therefrom.
Preferably, the frame of the switching device contains the cantilevered contact in one branch and the fusible conductor in the other branch. This cantilevered contact can be designed to swing transversely to the axis of the evacuation tube. This latter feature insures that the switching device compactly fits within a conventional screw base.